The present invention relates to a method for removing metals from air pollution control residues. More specifically, the present invention is concerned with a method for removing metals such as lead, cadmium and mercury from air pollution control residues.
The growing production of domestic and industrial wastes in industrialised countries causes serious disposal problems. A largely used technique for managing these wastes is incineration. Incineration produces ashes and hot gases. The hot gases generally contain volatilised metals and metals adsorbed on particles. These toxic metal components are produced during the combustion of wastes containing inert forms of these metals. Most incinerators contain air pollution control (APC) devices meant to treat hot gases and limit environment contamination.
Municipal solid waste incinerators (MSWI), for instance, may comprise three types of APC devices operating in sequence. The hot gases are first directed to boiler tubes wherein a portion of the metals contained in these gases precipitate and become adsorbed on fly ashes particles to produce boiler fly ashes. The residual gas stream travels through an electrostatic precipitator which traps a further portion of the remaining metal contaminants and forms electrostatic precipitator fly ashes. Finally, what remains of the gas stream travels through an injection of dry or semi-dry lime which retains acids and metals. Each of these devices thereby produces high metal content APC residues, namely and respectively boiler fly ashes, electrostatic precipitator fly ashes and used lime.
Thus, MSWI produce two main types of residues: bottom ashes and APC residues. The bottom ashes constitute between 20 and 35% of the initial waste mass and generally does not have a high content in toxic metals. They are generally therefore not considered hazardous. The APC residues constitute 2 to 3,5% of the initial waste mass and constitute hazardous wastes.
Various attempts have been made to treat APC residues in order to convert them into non-hazardous material. Attempts have been made to adapt techniques used for treating coal APC residues to MSWI APC residues. Coal APC residues are commonly mixed with coal combustion bottom ashes and used as fill material in building products such as brick, cement or road fill. These techniques however are not adequate for MSWI APC residues which contain much higher concentrations of toxic substances, namely heavy metals. See, Theis and Gardner in Environmental Assessment of ash disposal, CRC Reviews in Environmental control, vol. 20 p 21-42, 1990. One of the techniques currently used for MSWI APC residues is the stabilisation of APC residues in cement prior to landfill. Such method has the disadvantage of increasing the volume of APC residues to be landfilled and consequently entailing high costs.
Various techniques comprising the recuperation of valuable elements in the residues and their resale have also been proposed in attempts to reduce their net management costs. Attempts have been made to extract phosphate and metals from sewage sludges incinerators derived bottom and fly ashes. The principal value so extracted was the phosphate. Sewage sludges have a high phosphate content which justifies the costs of this technique. It may not be cost effective for the decontamination of APC residues derived from sources that have lower phosphate contents. For instance, MSWI APC residues contain 5 to 20 times less phosphate.
Others have proposed the extraction of iron and aluminium from fly ash generated during coal combustion methods to produce electricity. The iron and aluminium salts so extracted could be used in wastewater treatment. The extraction was conducted in two steps 1) NaOH (15%) at 90xc2x0 C. and 2) HCl (15%) at 90xc2x0 C. It may not be cost effective for the decontamination of APC residues derived from sources that have lower iron and aluminium content. For instance, MSWI APC residues contain amounts of iron and aluminium that would not justify the use of this technique. Furthermore, the APC residues and salts produced by this method still contain excessive amounts of toxic metals. For this reason, the salts are of low quality and the residues could not be directly landfilled.
A technique involving the extraction of metals from APC residues comprising mixing 5% of APC residues with HCl at pH 3 and NaCI 1 M was also proposed. This method is costly due to its high HCl consumption. Its efficiency is also limited because of the difficulties involved in the technique used for extracting the metals: selective plating of lead, zinc, cadmium, nickel or copper is hindered by the high iron concentration in solution.
Similarly, U.S. Pat. No. 4,649,011 involves the extraction of metals which have a high market value (vanadium, chromium, molybdenum, actinides and lanthanides) from combustion wastes from the coal industry. The method describes involves the addition of sodium and potassium carbonate to the combustion waste and heating the mixture at 60xc2x0 C. The combustion wastes are then treated in an acid solution having a pH of between 0 and 1. This method involves the use of a large quantity of alkaline and acid chemicals so that it is not cost effective for treating APC residues that do not contain these rare metals.
Vitrification of APC residues has also been proposed. This method requires that the bottom ashes be mixed with the APC residues so that it has the disadvantage of increasing the volume of wastes to dispose and consequently increasing the costs. Moreover the hot gas generated during the method is difficult to treat.
Other methods suggested use acid leaching techniques to dissolve metals contained in APC residues. U.S. Pat. No. 5,512,257 and Canadian Patent Application No. 2,053,443 by Frey, describe a method that uses the acidic water generated in a wet scrubber to extract metals. The wet scrubber is an APC device wherein the hot gas is put in contact with water thereby increasing its acid content. Wet scrubbers are not common APC devices. The efficiency of this method remains to be established. The fact that treated residues are returned in the combustion chamber for further stabilisation seems to indicate that they could not be directly disposed of and that their metal content remains excessive after the method. Canadian Patent No. 1,242,567 by Vogg, also describes an acid leaching of the APC residues in wet scrubber acid streams.
U.S. Pat. No. 4,524,049 describes a method that comprises an acid extraction technique at high temperature and pressure to recover nickel and vanadium from fluid coke or fly ash derived from the burning of fluid coke.
U.S. Pat. No. 5,453,111 describes a method to recover iron, zinc, cadmium and lead from electric arc furnace dust. The dust is first roasted and then treated with an ammonium chloride solution at high temperature (90xc2x0 C.).
International Application No. WO09744500 describes a method by which APC residues are mixed with water to produce a solution having a pH higher than 11.8. This pH is reached without the addition of any alkaline chemical. Alkaline washes of APC residues are generally not sufficient to substantially remove the toxic metals that they contain.
These methods suffer from important drawbacks. Some of them require large quantities of expensive chemicals such as acids or oxidizers or involve extreme conditions requiring costly equipments. Others involve high costs because of the large volume of treated residues produced that needs to be landfilled. Hence, the methods requiring that bottom ashes be mixed with APC residues constitute expensive methods. Finally some of the methods described do not appear to efficiently remove toxic metals or diminish the leaching potential of the APC residues.
The present invention provides a method for the treatment of air pollution control (APC) residues comprising the steps of a) washing the residues at least one time in an alkaline solution maintained at a pH not lower than about 11.5; b) separating the alkaline washed residues from the alkaline solution produced in step a); c) verifying whether the alkaline washed residues obtained in step b) are substantially devoid of metal components. When the alkaline washed residues obtained in step b) are not substantially devoid of metal components, the alkaline washed residues are washed at least one time in an acid solution maintained at a pH between about 2 and about 4.5 and the acid washed residues are separated from the acid solution so as to produce residues substantially devoid of metal components and d) recover these residues.
The present invention provides an alternative method for the treatment of air pollution control (APC) residues comprising the steps of a) washing the residues at least 1 to 6 times in an alkaline solution maintained at a pH of not lower than 11.5 for a period of between about 30 seconds to about 30 minutes for each wash; b) separating the alkaline washed residues from the alkaline solution produced in step a); c) verifying whether the alkaline washed residues obtained in step b) are substantially devoid of metal components. When the alkaline washed residues obtained in step b) are not substantially devoid of metal components, the alkaline washed residues are washed at least 3 to 7 times in an acid solution maintained at a pH between about 2 and about 4.5 so as to produce residues substantially devoid of metal components and d) recover those residues.
The present invention provides an alternative method for the treatment of air pollution control (APC) residues comprising the steps of a) washing the residues at least one time in an alkaline solution maintained at a pH of not lower than about 11.5 for a period of between about 30 seconds to about 30 minutes for each wash; b) separating the alkaline washed residues from the washing solution produced in step a); c) washing the alkaline washed residues from step b) at least 3 times in an acid solution maintained at a pH between about 2 and about 4.5; d) separating the acid washed residues from the washing solution produced in step c) so as to obtain residues substantially devoid of metal components; e) recover these residues and wherein metals may be recuperated from the washing solution produced in steps b) and d).
The alkaline and acidic washing steps can be conducted in different reactors or alternatively in the same reactor operated as a sequential batch reactor.
The method of the present invention can be used to treat APC residues originating from various combustion facilities such as municipal solid waste incinerators (MSWI), hazardous waste incinerators, sewage sludges incinerators, coal burning incinerators, medical waste incinerators or metallurgical plants. The method of the present invention is also useful for treating APC residues which have undergone a pre-treatment such as the removal of organic contaminants.
In other specific embodiments of the present invention, the method described herein could be applied to a specific granulometric fraction of the APC residues.
The methods according to the present invention encompass the use of various techniques to concentrate the metals present in the liquid. For instance, the techniques of electrowinning, electroplating, cementation, adsorption of metals on various matrix, etc . . . are appropriate techniques that could be used within the method of the present invention.
The number of washes applied according to the present invention is dependent of the nature and concentration of the metal contaminants.
The method according to the present invention could be performed in a permanent or a temporary plant. It could also be used on a mobile unit installed on water or on land.
Additional steps can be added to the method according to the present invention without departing from its spirit and nature. For instance, after the acid washes the APC residues and/or metal concentrates can be subjected to other treatments to better prepare them for the purpose for which they are intended. For instance, the APC residues may be intended for uses in as construction material (concrete, brick, road fill), in sanitary landfill (as daily covering material), etc.
As provided herein the expression xe2x80x9cair pollution control residuesxe2x80x9d (APC residues) is meant to include boiler fly ashes, electrostatic precipitator fly ashes, used lime or a mixture thereof.
As provided herein, the expression xe2x80x9csubstantially devoid of metal componentsxe2x80x9d means that the level of metal contained is sufficiently low to constitute non-hazardous waste or material. In particular, according to the year 2000 United States norms, residues that leach less than 5 mg/L of lead, less than 1 mg/L of cadmium and less than 0.2 mg/L of mercury during TCLP tests are considered to be substantially devoid of metal components.
Other objects, advantages and features of the present invention will become more apparent upon reading of the following non-restrictive description of preferred embodiments thereof, given by way of example only with reference to the accompanying drawings.